A solid oxide fuel cell is a fuel cell employing as an electrolyte a solid electrolyte having oxygen ion conductivity, and attracts attention as a clean energy since the electrochemical reaction which causes electromotive force is a hydrogen oxidation reaction, and no carbon dioxide gas is formed. A solid oxide fuel cell usually has a stack structure comprising single cells each comprising an air electrode as an oxide, a solid electrolyte and a fuel electrode connected by an interconnector. Its operating temperature is usually about 1,000° C., and decrease in the temperature is attempted and practically employed by various studies, however, it is at least about 600° C. and is still high temperature.
An air electrode material constituting the air electrode of a solid oxide fuel cell, is basically required to have such properties that (1) it has a high oxygen ion conductivity, (2) it has a high electron conductivity, (3) its thermal expansion is similar to or about the same as that of an electrolyte, (4) it has high chemical stability and has high compatibility with other constituting materials, and (5) the sintered product is required to be a porous product and it has a certain strength, etc.
Patent Document 1 proposes as an air electrode material constituting an air electrode, a ceramic powder containing as the main component a lanthanum ferrite perovskite oxide represented by the compositional formula (L1−xAEx)1−y(FezM1−z)O3+δ, wherein L is one or more of elements selected from the group consisting of Sc, Y and rare earth elements, AE is one or two of elements selected from the group consisting of Ca and Sr, M is one or more of elements selected from the group consisting of Mg, Sc, Ti, V, Cr, Co and Ni, 0<x<0.5, 0<y≦0.04 and 0≦z<1.
Patent Document 1 discloses in Example 2 (La0.6Sr0.4)1−z(Co0.2Fe0.8)O3+δ(y=0.02, 0.04) prepared by a citrate method. The (La0.6Sr0.4)1−z(Co0.2Fe0.8)O3+δ is a sample prepared by the same method as in Comparative Example 1 in this specification, and is inferior in the homogeneity of the constituting elements.
Further, Patent Document 2 proposes a lanthanum ferrite perovskite oxide of (La1−xSrx)aCoyFe1−yO3 (I)(0.2≦x≦0.5, 0.1≦y≦0.6, 0.9≦a≦1.0), and a method for producing the lanthanum ferrite perovskite oxide using citric acid and an ammonia compound such as ammonium bicarbonate or ammonium carbonate.
The present inventors have tried to prepare (La1−xSrx)aMnO3+δ composite oxide powder having a perovskite structure by using citric acid and an ammonia compound such as ammonium bicarbonate or ammonium carbonate as disclosed in Patent Document 2, however, found industrial application of the production method difficult due to a high cost since an expensive apparatus to detoxify harmful ammonia gas and/or nitrogen oxide gas which forms at the time of firing is necessary.
Further, as a material which satisfies properties of an air electrode, a composite oxide (sometimes referred to as LSM) represented by (La1−xAx)1−αMnO3+δ (wherein A is strontium and/or calcium) having a perovskite structure is energetically studied and developed as an air electrode material excellent in the electrode activity.
For example, Non-Patent Document 1 discloses that (La1−xSrx)0.94 MnO3−δ(0.08≦x≦0.21) which is A site deficient LSM undergoes phase transition together with oxygen release at about 1,000° C. in the air. In this Non-Patent Document 1, a sample used for heat analysis and electrical conductivity measurement is prepared by a typical solid phase method of mixing material powders which are carbonates by a mixing and pulverizing machine, firing the mixture at 1273K for 15 hours, pulverizing the fired product and further firing the pulverized product at high temperature of 1573K for 48 hours. However, by such a solid phase method, lanthanum, strontium and manganese as elements constituting LSM are hardly homogeneously dispersed in particles even by firing at a temperature so high as 1573K (1,300° C.).
Further, Patent Document 3 discloses in paragraph [0024] as a method for preparing La0.8Sr0.2MnO3+δ a method of adding a mixture of nitrates as starting materials in ethanol in which oxalic acid is dissolved to coprecipitate an oxalate, followed by calcination. Since in this coprecipitation method precipitates are formed from a uniform solution, it is apparently considered that one having a uniform composition is easily formed, however, according to studies by the present inventors, it was found that in practice, the precipitates do not have a uniform composition since the pH at which insoluble salts of the respective elements precipitate and their crystal growth rates are different among the nitrates of the three types of elements. For example, a salt of one element is precipitated first and grows into large particles, and then micro crystals of the next element are precipitated on the large particles, and accordingly it is difficult in principle to obtain precipitates having a sufficiently uniform composition.